If what you want is a quotation or offer, then sending a letter of intent makes you look ignorant while requesting a quotation will generally at least make you look like you know what it is that you want. And what you want, after all, is a price quote. Hence you want to request a price quote, via a request for quotation.
Among Internet based brokers and agents the LOI is commonly advanced as a leading inquiry document. This protocol is utterly incorrect, and misuse of the LOI is often seen by experienced Exporters as a mark of an amateur. Letters of Intent are not standard international business documents, though they do legitimately pop up rarely, much less often than most brokers believe.